The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 86a82ae0b5095ea24c55898a3f025791e7958b21 Gitweb: https://git.kernel.org/tip/86a82ae0b5095ea24c55898a3f025791e7958b21 Author: Thomas Gleixner tglx@linutronix.de AuthorDate: Wed, 23 Sep 2020 17:46:20 +02:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Wed, 23 Sep 2020 22:44:56 +02:00

x86/ioapic: Unbreak check_timer()

Several people reported in the kernel bugzilla that between v4.12 and v4.13 the magic which works around broken hardware and BIOSes to find the proper timer interrupt delivery mode stopped working for some older affected platforms which need to fall back to ExtINT delivery mode.

The reason is that the core code changed to keep track of the masked and disabled state of an interrupt line more accurately to avoid the expensive hardware operations.

That broke an assumption in i8259_make_irq() which invokes

disable_irq_nosync(); irq_set_chip_and_handler(); enable_irq();

Up to v4.12 this worked because enable_irq() unconditionally unmasked the interrupt line, but after the state tracking improvements this is not longer the case because the IO/APIC uses lazy disabling. So the line state is unmasked which means that enable_irq() does not call into the new irq chip to unmask it.

In principle this is a shortcoming of the core code, but it's more than unclear whether the core code should try to reset state. At least this cannot be done unconditionally as that would break other existing use cases where the chip type is changed, e.g. when changing the trigger type, but the callers expect the state to be preserved.

As the way how check_timer() is switching the delivery modes is truly unique, the obvious fix is to simply unmask the i8259 manually after changing the mode to ExtINT delivery and switching the irq chip to the legacy PIC.

Note, that the fixes tag is not really precise, but identifies the commit which broke the assumptions in the IO/APIC and i8259 code and that's the kernel version to which this needs to be backported.

Fixes: bf22ff45bed6 ("genirq: Avoid unnecessary low level irq function calls") Reported-by: p_c_chan@hotmail.com Reported-by: ecm4@mail.com Reported-by: perdigao1@yahoo.com Reported-by: matzes@users.sourceforge.net Reported-by: rvelascog@gmail.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Tested-by: p_c_chan@hotmail.com Tested-by: matzes@users.sourceforge.net Cc: stable@vger.kernel.org Link: https://bugzilla.kernel.org/show_bug.cgi?id=197769 --- arch/x86/kernel/apic/io_apic.c | 1 + 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c index 779a89e..21f9c7f 100644 --- a/arch/x86/kernel/apic/io_apic.c +++ b/arch/x86/kernel/apic/io_apic.c @@ -2243,6 +2243,7 @@ static inline void __init check_timer(void) legacy_pic->init(0); legacy_pic->make_irq(0); apic_write(APIC_LVT0, APIC_DM_EXTINT); + legacy_pic->unmask(0);

unlock_ExtINT_logic();